Processes of smelting finely divided iron ore



March 3, 1959 w. E. GREENAWALT PROCESSESOF SMELTING FINELY DIVIDED IRON ORE Filed Oct. 24, 1957 1 N VEN TOR United States Patent PROCESSES OF SMELTING FINELY DIVIDED IRON ORE William E. Greenawalt, Denver, Colo. Application October 24, 1957, Serial No. 692,072 4 Claims. (Cl. 75-40) My invention relates to process of smelting finely divided iron ore, such as that obtained from milling ore too low in grade to be profitably smelted direct. It relates more particularly to an improvement or modification of my co-pending applications, Serial No. 612,494, filed September 27, 1956, and Serial No. 656,375, filed May 1, 1957. It will be described in its application to low grade magnetic and non-magnetic ores or a mixture of both.

Iron ore concentrate, usually obtained by gravity, flotation, or magnetic separation of finely ground low grade ore, is ordinarily obtained by repeated fine grinding, and prepared for blast furnace smelting by pelletizing and sintering which converts it into semi-fused lumps. This is done by mixing the concentrate with about ten percent water and several percent of fine coke, igniting the charge of the mixture at its surface, and passing a blast of air downwardly through the porous charge. The ore, in sintering, is heated to about 2200 deg. F., after which it is cooled to atmospheric temperature of about 65 deg. F. When the cold sinter is charged into the blast furnace it has to be reheated through the sintering temperature of about 2200 deg. F. to the smelting temperature of 2800-3000 deg. F. This involves a heavy expense of both installation and operation. It is evident that if direct smelting of the fine concentrate could be greatly cheapened it would be practical to economize in both milling and smelting.

It has been repeatedly proposed to get these results by showering the finely ground concentrate through a highly heated reducing atmosphere of a vertical shaft smelting furnace, but the difficulties involved have not met with encouraging results in practical operation, principally among which are very fine grinding in milling, and reoxidation of the reduced ore after smelting.

Low grade ore may be magnetic, non-magnetic, or a mixture of both. A preliminary mill concentration of some sort, of the coarsely ground ore is usually necessary to cheaply eliminate the barren or worthless gangue from the ore. Magnetic ore containing about 25% iron may be separated, after coarse grinding to -20 mesh, to a concentrate containing about 40% iron with the elimination of one third as worthless gangue. If the ore is non-magnetic, or a mixture of magnetic and nonmagnetic oxide, the usual procedure has been to roast the coarsely ground ore in a reducing atmosphere to convert the non-magnetic oxide into the magnetic oxide, and then subject the reduced roasted ore to magnetic separation which, for the purpose of illustration may be assumed, as in the case of naturally occurring magnetite, to give about a 40% iron concentrate.

In either case, in present practice, the coarsely ground ore concentrate is ground to an impalpably fine powder, and subjected to magnetic separation to produce a high grade, 60% concentrate, which is then pelletized and sintered for blast furnace smelting in the usual way.

Roasting non-magnetic ore to convert it into magnetic form is a delicate operation. Practical reduction requires a temperature of about 1700 deg. F., and has to be confined within narrow limits. Reduction appreciably below that temperature is too slow to be of practical value, and if conducted appreciably above that temperature there is danger of fusion, which would make the roasted ore worthless for mechanical or milling separation.

2,876,093 Patented Mar. 3, 1959 In addition, the roast must be uniform, which implies some sort of agitation, with the production of the usual roasting dust.

The object of my invention is to cheapen milling, avoid pelletizing and sintering, dispense with hot blast stoves used in regular blast furnace smelting, and to a very large extent supplantiug expensive blast furnace coke with cheaper fuel. In addition, on non-magnetic ores, to dispense with pre-roasting to convert the non-magnetic oxide into the magnetic oxide.

The preferred method of operating the process will now be described by referring to the accompanying drawing. The finely divided ore, usually wet, is delivered into the bin 1 and fed in a regulated stream into the drier 2, where it is advanced while drying into the receiving hopper 3, from which it is showered in a regulated stream through the highly heated atmosphere of the shaft melting furnace 4 to melt it and to produce a mixture of melting furnace gas, slag, and reduced and unreduced molten ore. This mixture of melting furnace gas, slag, and reduced and unreduced molten ore, flows in a continuous stream from the melting furnace 4 into a separating chamber 5 Where the melting furnace gas is separated from the mixture of slag, reduced and unreduced molten ore. The reduced molten ore, in the form of metallic iron, is separated by gravity from the slag and unreduced ore, and flows out of the separating chamber 5 through the tap hole 21. The slag and unreduced ore flows through the duct 6 into the smelting furnace 7 filled with coal or coke in permeable lump form, or other reducing agent for iron oxide, much the same as in regular blast furnace smelting, except that the ore would be charged molten, and contain only a comparatively small amount of unreduced iron oxide of the original ore, and that the upper section of the column of carbon would have to be maintained at, or above, the melting point of the molten ore. Air may be injected into the smelting furnace through the tuyeres 17, and a mixture of air and fluid fuel may be injected into the upper section of the carbon through the burners 22. The hot smelting furnace gas containing carbon monoxide flows through the duct 16 into the melting furnace 4. The slag from the smelting furnace 7 flows out through the slag hole 8, and the molten pig iron through the tap hole 9.

The melting furnace gas, separated from the Slag and reduced and unreduced ore, flows from the separating chamber 5 into a dust coalescing chamber 10, where the molten dust is coalesced into liquid form and automatically flows back into the separating chamber. The permeable filling of the coalescing chamber may be heated as desired to keep the ore in molten condition to coalesce it, through the fuel burners 19. The coalescing chamber filling is charged into the coalescing chamber through the feeder 11. The gas issuing from the coalescing chamber, freed from dust, flows into the heat exchanger 12, to heat fresh air, passed through it by means of the duct 15, to be used anywhere in the process. The waste melting furnace gas issuing from the heat exchanger is forced through the conduit 18 by means of the blower 13 to dry or to heat fresh ore.

In describing the operation of the process it may be assumed, merely for illustration purpose, that the mine ore, whether naturally magnetic or made so through pre-roasting, is milled to produce a medium grade, 40%, concentrate, by grinding to 10 mesh. So ground only about 10 to 15 percent of the ore will be of 10 mesh screen size. About percent may be finer than 20 mesh, and a large percentage will be finer than mesh. When this ore is showered through the highly heated reducing atmosphere of the melting furnace, the

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finer particles will be instantly reduced and melted. Probably 80 to 90 percent will be reduced and melted as the molten ore flows from the melting furnace into the separating chamber. The remaining to per-' cent would in any case be highly heated and partly reduced. Melting and reducing are largely controlled by the temperature of the melting furnace and the concentration of the reducing agent; both of which are variable factors, and will depend largely on the height of the melting furnace. It costs very little more to elevate the ore 100, or even 150, feet, than to elevate it feet. Reduction may take place at a lower temperature than melting. At the high temperature of the melting furnace they will be simultaneous.

The molten ore as it flows from the melting furnace into the separating chamber will consist of a mixture of slag, a very large percentage of reduced molten ore in the form of metallic iron, and a small percentage of partly fused and partly unreduced iron, where it is mixed with the pool of molten ore in the separating chamber to further melt and reduce it, while at the same time the molten metallic iron is separated from the slag and unreduced ore, and removed through the tap hole 21. The atmosphere and the temperature of the separating chamber may be controlled by means of the burner 20.

The molten ore from the separating chamber, consisting of slag and a small percentage of unreduced iron oxide, at a temperature of about 2300 deg. F., and separated from the melting furnace gas, flows into and through the highly heated column of carbon or other reducing agent in the smelting furnace, Where the recovery of the iron can be made almost complete, due to high temperature and highly concentrated reducing agent. The recovery of the iron can be made more complete than in regular blast furnace smelting, principally because the charge is leaner in iron oxide and the concentration of the reducing agent is higher, and the temperature of the furnace is under better control. Only about 50 percent of the original ore is passed through the smelting furnace, due mostly to the previous removal of most of the iron and oxygen composing the original ore. The hot smelting furnace gas, high in carbon monoxide, is delivered into the melting furnace or into the separating chamber. The fuel burners 22 serve to keep the surface temperature of the carbon in the smelting furnace 4 high enough to prevent chilling of the molten ore as it flows from the melting furnace into the smelting furnace.

This method of procedure permits of coarser fine grinding than is practical when all of the ore is to be reduced in a shower shaft melting furnace, because the time of descent in a shower shaft furnace is very short, and coarse particles would not be entirely melted and reduced within the practical time limit. If, for example, a cubic inch of ore is ground to 10 mesh it would be divided into no less than 1,000 particles. If ground to 100 mesh it would be divided into no less than 1,000,000 particles. If ground to 300 mesh, as with ore in which the iron and gangue are in exceedingly intimate contact, it would be divided into no less than 27,000,000 particles. The taconite ore in the Lake Region is nearly three times as hard as limestone and twice as hard as granite and is not easily ground exceedingly fine. The hardness of ore ground to 10 mesh, and melted and smelted, as in the present process, is not harmful, and may be beneficial due to producing less dust.

Extra flux and fuel are required to smelt a medium grade, say 40%, concentrate over that required to smelt a concentrate, but the fuel used in the melting furnace may be the cheaper and more convenient powdered coal, fuel oil, or natural gas, as compared with the more expensive coke used in regular blast furnace smelting, and because only about half of the ore is charged into the smelting furnace at a temperature of about 2300 deg. F. as compared with all of it at deg. F.

Over half of the fuel used in regular blast furnace smelting is consumed in heating the cold ore to the smelting temperature, and the amount of carbon consumed in reducing reactions in the smelting furnace is proportional to the amount of iron oxide reduced. Molten iron oxide has a very corrosive effect on the furnace refractories. This is reduced to a minimum with the quick simultaneous reduction and melting of the showered ore in the melting furnace and the leaner ore and highly concentrated reducing agent in the smelting furnace.

I claim:

1. A process of smelting finely divided iron oxide ore comprising, showering the ore through the highly heated atmosphere of a vertical shaft furnace to melt it and to produce a mixture of molten slag, reduced ore in the form of metallic iron and unreduced ore in the form of iron oxide, separating the reduced metallic iron from the mixture of slag and unreduced iron oxide, then passing the mixture of slag and unreduced iron oxide into and through a highly heated permeable column of carbon to reduce the remaining unreduced iron oxide into metallic iron, and separating the reduced molten metallic iron from the molten slag.

2. A process of smelting finely divided iron oxide ore comprising, showering the ore through the highly heated atmosphere of a vertical shaft furnace to melt it and to produce a mixture of melting furnace gas, slag, and reduced and unreduced molten ore, separating the melting furnace gas from the mixture of slag and reduced and unreduced molten ore, separating the reduced molten ore in metal form from the mixture of slag and unreduced ore in oxide form, then passing the mixture of slag and unreduced ore in oxide form through a highly heated -mass of reducing agent to complete the reduction of the iron oxide to metallic iron, and separating the molten metallic iron from the molten slag.

3. A process of smelting finely divided iron oxide ore comprising, showering the ore through a highly heated atmosphere of a vertical shaft furnace to melt it and to produce a mixture of reduced ore in the form of metallic iron and unreduced ore in the form of iron oxide, with drawing the mixture of slag, reduced and unreduced molten ore, and melting furnace gas, in a combined continuous stream from the melting furnace, separating the melting furnace gas from the mixture of slag, reduced and unreduced ore, separating the reduced molten ore in metal form from the mixture of slag and unreduced ore in oxide form, then passing the mixture of molten slag and unreduced oxide ore through a highly heated permeable column of carbonaceous material to complete the reduction of the iron oxide, and separating the molten metallic iron from the molten slag.

4. A process of smelting finely divided iron oxide ore comprising, showering the ore through the highly heated atmosphere of a shaft furnace to melt it and to produce a mixture of reduced ore in the form of metallic iron and unreduced ore in the form of iron oxide, delivering the resulting mixture of slag, reduced and unreduced molten ore, and melting furnace gas, in a combined con tinuous stream into an enclosed chamber, separating the melting furnace gas from the mixture of slag, reduced and unreduced molten ore, separating the reduced molten ore in the form of metallic iron from the mixture of slag and unreduced ore in oxide form, then passing the mixture of slag and unreduced iron oxide through a mass of highly heated reducing agent in a smelting furnace to complete the reduction of the iron oxide into metallic iron, separating the molten metallic iron from the molten slag, and delivering the hot gas issuing from the smelting furnace into the melting furnace.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A PROCESS OF SMELTING FINELY DIVIDED IRON OXIDE ORE COMPRISING, SHOWERING THE ORE THROUGH THE HIGHLY HEATED ATMOSPHERE OF A VERTICAL SHAFT FURNACE TO MELT IT AND TO PRODUCE A MIXTURE OF MOLTEN SLAG, REDUCED ORE IN THE FORM OF METALLIC IRON AND UNREDUCED ORE IN THE FORM OF IRON OXIDE, SEPARATING THE REDUCED METALLIC IRON FROM THE MIXTURE OF SLAG AND UNREDUCED IRON OXIDE, THEN PASSING THE MIXTURE OF SLAG AND UNREDUCED IRON OXIDE INTO AND THROUGH A HIGHLY HEATED PERMEABLE COLUMN OF CARBON TO REDUCED THE REMAINING UNREDUCED IRON OXIDE INTO METALLIC 